This RSS feed can have at max 25 posts. So at any point you will have 25 links in your sitemaps.In case you want to have full sitemap of your entire site with all links in it so that you can easily see out of how many are indexed by google; just add

atom.xml?redirect=false&start-index=1&max-results=500

as sitemap in Google webmaster tools.

Best Site map for blogger

This will have 500 post links of your site which is more than enough. (You can add another sitemap with start-index=501 and max-results=500 in case you have thousands of posts )